RFR: 8365052: G1: Remove G1CollectionSet::groups() accessors

[Thomas Schatzl]

Hi all,

  please review this small change that remove the public `G1CollectionSet::groups()` accessors because they are never used from outside the `G1CollectionSet` class.

Testing: local compilation, gha

Thanks,
  Thomas

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Depends on: https://git.openjdk.org/jdk/pull/26670

Commit messages:
 - 8365052

Changes: https://git.openjdk.org/jdk/pull/26688/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26688&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365052
  Stats: 5 lines in 3 files changed: 0 ins; 3 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/26688.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26688/head:pull/26688

PR: https://git.openjdk.org/jdk/pull/26688